Oxidation-stable hydraulic oil

ABSTRACT

A synthetic ester comprising a reaction product of: (a) a polyol corresponding to formula I:  
                 
 
     wherein R 1  is a hydrogen atom, a methyl group or a hydroxyl group and R 2  and R 3  independently of one another, represent a methyl group or a group CH 2 —OH, with the proviso that at least two hydroxyl groups are present in the molecule, and (b) an unsaturated fatty acid having from about 16 to 18 carbon atoms, and wherein the synthetic ester has an iodine value of from about 65 to 100, a saponification value of from about 160 to 200, and an acid value of at most 3.

[0001] This invention relates to oxidation-stable hydraulic oils containing certain synthetic esters based on polyols and unsaturated fatty acids and to the use of certain additive mixtures for improving the oxidation stability of hydraulic oils.

[0002] Hydraulic systems are used in a variety of motorized vehicles, for example automobiles, trucks, cranes, trains and other transport media, and in agricultural machinery, ships and in factories and in the railroad sector. The hydraulic systems generally contain a hydraulic fluid which in turn traditionally contains petrochemical fluids or esters or other oleochemicals as the basic fluid. Preference is increasingly being given to oleochemicals by virtue of their biodegradability.

[0003] In this connection, polyol esters of fatty acids, preferably unsaturated fatty acids, are particularly well-known from the prior art as suitable basic fluids for hydraulic oils. Thus, WO 97/39086 discloses esters obtained by reaction of polyols, including trimethylolpropane, with mixtures of fatty acids, the ratio of short-chain fatty acids to the long-chain fatty acids having to be in the range from 2:1 to 1:20. According to the cited document, the claimed esters are supposed to have advantageous technical properties by virtue of their stability at low temperatures.

[0004] Besides stability at low temperatures, however, ester systems and hydraulic fluids are also expected to show high oxidation stability which comes into play in particular when the hydraulic oil is exposed to high temperatures in the presence of atmospheric oxygen. In order to achieve this, antioxidants are added to hydraulic oils as typical additives in the prior art. A crucial factor in this case is the so-called modified dry turbine oxidation stability test (“dry TOST” for short) according to DIN 51587 which tests the stability of test oils on ageing at 95° C. in the presence of oxygen. Conventional systems reach the critical limit of 2.0 mg KOH/g test oil after only 170 to about 300 hours. However, since increasingly higher oxidation stability is expected, there is a constant demand in industry for the provision of more oxidation-stable oils. Attempts to achieve the high oxidation stability required by adding other antioxidants have not yet proved successful.

[0005] Accordingly, the problem addressed by the present invention was to provide oxidation-stable fluids for use in hydraulic oils which would meet present requirements, even in regard to their biodegradability. It has surprisingly been found that selected polyol esters satisfy the requirements stated above.

[0006] Accordingly, the present invention relates to synthetic esters of

[0007] a) polyols corresponding to general formula (I):

[0008] in which R¹ is a hydrogen atom, a methyl group or a hydroxyl group and R² and R³ independently of one another represent a methyl group or a group CH₂—OH, with the proviso that at least two hydroxyl groups must be present in the molecule, and

[0009] b) unsaturated fatty acids containing 16 to 18 carbon atoms,

[0010] characterized in that the synthetic esters have an iodine value (to DGF—C—V 11b) of 65 to 100, a saponification value (to DGF C—V 3) of 160 to 200 and an acid value (to DGF C—V 2) of at most 3).

[0011] The synthetic esters according to the invention are prepared in known manner by reaction of the polyols with the unsaturated fatty acids in the presence of suitable catalysts and at elevated temperature. The reaction products may then be removed from the mixture by distillation. Synthetic esters which have a kinematic viscosity at 40° C. of 40 to 70 mm²/sec. and a kinematic viscosity at 100° C., as measured to DIN 51562, of 5 to 20 mm²/sec. are particularly preferred. The polyols used for the preparation of the synthetic esters are preferably selected from the group consisting of neopentyl glycol, trimethylol propane, trimethylolethane and/or pentaerythritol. In a particularly preferred embodiment, trimethylol propane is used as the starting point for the production of the synthetic esters according to the invention. The C₁₆₋₁₈ fatty acids which are reacted with the polyols a) to form the synthetic esters are preferably derived from oleic acid in its various qualitites. The esters according to the invention are further distinguished by the fact that their acid value, as measured to DGF C—V2, is at most 3 and preferably at most 2mg KOH/g. The iodine value, as measured to DGF C—V11B, should preferably be in the range from 65 to 100, more particularly in the range from 65 to 90 and most particularly in the range from 70 to 90 mg KOH/g. The saponification values as measured to DGF C—V should preferably be below 200, more particularly in the range from 160 to 190 and most particularly in the range from 170 to 185 mg KOH/g. The cloud point of the esters according to the invention, as measured to DGF D-III 3, is preferably in the range from −40 to −20° C. The water content, as measured to DGF C-III 13a (Karl Fischer method), is preferably at most 0.1 more particularly in the range from 0.01 to 0.05. Other characteristics of the esters according to the invention include the pour point (DIN ISO 3016) which is preferably at most −30° C. The cloud point to DIN EN 23015 is preferably at most −20° C. The esters according to the invention are not only oxidation-stable, they also remain stable for several years at normal storage temperatures.

[0012] The present invention also relates to a process for the production of synthetic polyol and complex esters in which polyols corresponding to general formula (I) are reacted under nitrogen with so-called oleins for at least 3 hours at temperatures of >200° C., after which the excess fatty acid is removed by distillation under reduced pressure. The oleins used in the process according to the invention are technical mixtures of unsaturated and saturated fatty acids which predominantly contain oleic acid. Such oleins are obtained from naturally occurring fats and oils, more particularly animal oils. The oleins obtained by the process according to DE 197 36 737 A1 are preferably used for the production of the esters according to the invention. Particular reference is made here to Examples 1, 3, 4, 7, 8 and 9 of DE 197 36 737 A1 which are hereby expressly included in the disclosure of the present application. According to the invention, oleins of the type in question have an iodine value as measured to DGF C-V11B in the range from 60 to 130 and a cloud point as measured to DGF C III of less than 11° C. In addition, the oleins used in the process according to the invention must contain more than 65% by weight oleic acid and less than 20% by weight linoleic acid.

[0013] The present invention also relates to a hydraulic oil containing 90 to 99% by weight, preferably 95 to 99% by weight and more particularly 97 to 99% by weight of a synthetic ester as described in the foregoing and 1 to 10% by weight of additives.

[0014] The additives belong to the classes known in principle to the expert, i.e. antioxidants, extreme-pressure (EP) and anti-wear (AW) additives, corrosion inhibitors, demulsifiers and/or defoamers. Nonferrous metal deactivators may also be present. The additives are present in the usual quantities, but in all in quantities of at most 10% by weight and preferably 1 to 3% by weight, based on the total weight of the hydraulic oil. The EP/AW additives are present in the hydraulic oils according to the invention in quantities of preferably 0.2 to 2.0% by weight, antioxidants in quantities of 0.2 to 1.0% by weight, corrosion inhibitors in quantities of 0.05 to 0.2% by weight, nonferrous metal deactivators in quantities of 0.05 to 0.5% by weight and antifoam additives (defoamers) in quantities of 0.005 to 0.04% by weight. Corresponding hydraulic oils are formulated by mixing the basic fluid with the additives, optionally at elevated temperature.

[0015] In one particularly advantageous embodiment, an additive containing alkylamines, tolyltriazole, alkyl phosphate and/or succinic acid semiesters and derivatives thereof is added to the hydraulic oils according to the invention. Additives such as these are preferably present in quantities of, in all, 0.5 to 2% by weight. A most particularly preferred embodiment is characterized by the use of an additive mixture which is commercially available under the name of Additin RC 9308. This additive contains C₁₂₋₁₄ tertiary alkyl amines, tolyltriazole, tri-n-butyl phosphate and tetrapropylene succinic acid semiester. The combination of this additive with the synthetic hydraulic oil gives a particularly oxidation-stable end product. Accordingly, the present invention also relates to the use of such an additive mixture for improving the oxidation stability of the polyol esters described in the foregoing.

[0016] The esters according to the invention and the hydraulic oils formulated from them show surprisingly high stability in the TOST test, more particularly the so-called “dry” TOST test where no water is present.

EXAMPLES

[0017] 1. Preparation of a TMP Ester According to the Invention

[0018] 14.5 kg of trimethylolpropane were heated and stirred under nitrogen for 4 hours at 250° C. with 85.5 kg of an olein prepared by the process according to DE 197 36 737 A1. The pressure was then reduced to less than 20 mbar and the excess fatty acid was removed.

[0019] The TMP ester had the following characteristics: acid value (DGF C-V 2):  2 mg KOH/g saponification value (DGF C-V 3): 187 mg KOH/g iodine value (DGF C-V 11b):  83 g/100 g kin. viscosity at 40° C. (DIN 51562):  42 mm²/s kin. viscosity at 1000° C. (DIN 515662):  9 mm²/s

[0020] 2. Performance Testing of the TMP Esters According to the Invention

[0021] A hydraulic oil containing 98% by weight of the TMP ester of Example 1 and 2% by weight of an additive mixture containing C₁₂₋₁₄ alkylamines, tolyltriazole, tri-n-butyl phosphate and tetrapropenyl succinic acid alkyl semiester (Additin RC 9308, a product of Rhein-Chemie) was tested.

[0022] The above hydraulic oil was subjected to a modified version of the turbine oxidation stability test (“TOST”) to DIN 51587 in order to determine the oxidation stability of the hydraulic oil. By way of modification of DIN 51587, the test was carried out in the absence of water. The water was replaced by the same quantity of test oil. In the test to DIN 51587, the test oil is aged under under nitrogen at 95° C. in the presence of steel and copper wire. The end point of the ageing process is either reached at a neutralization value of 2.0 mg KOH/g oil or, if this value has not been reached after 1000 h, the value measured after 1000 h is taken as the result. In the case of the hydraulic oil according to the invention, the neutralization value after 1000 h was 1.9. A neutralization value of 2.0 and hence the end of the ageing process was only reached after 1414 h. 

1. Synthetic esters of a) polyols corresponding to general formula (I):

in which R¹ is a hydrogen atom, a methyl group or a hydroxyl group and R² and R³ independently of one another represent a methyl group or a group CH₂—OH, with the proviso that at least two hydroxyl groups must be present in the molecule, and b) unsaturated fatty acids containing 16 to 18 carbon atoms, characterized in that the synthetic esters have an iodine value (to DGF—C—V 11b) of 65 to 100, a saponification value (to DGF C—V 3) of 160 to 200 and an acid value (to DGF C—V 2) of at most 3):
 2. Synthetic esters as claimed in claim 1, characterized in that the esters have a kinematic viscosity at 40° C. of 40 to 70 mm²/s² and a kinematic viscosity at 100° C. (as measured to DIN 51562) of 5 to 20 mm²/s².
 3. Synthetic esters as claimed in claims 1 to 2, characterized in that the polyol a) is selected from the group consisting of neopentyl glycol, trimethylol propane, trimethylol ethane and/or pentaerythritol.
 4. Synthetic esters as claimed in claims 1 to 3, characterized in that component b) is oleic acid.
 5. A process for the production of synthetic polyol esters in which polyols corresponding to general formula (I) in claim 1 are reacted under nitrogen with oleins for at least 3 hours at temperatures of >200° C., after which the excess fatty acid is removed by distillation under reduced pressure, characterized in that the oleins have an iodine value (DGF C—V11B) in the range from 60 to 130 and a cloud point (DGF D III) of less than 11° C. and contain more than 65% by weight oleic acid and less than 20% by weight linoleic acid.
 6. A hydraulic oil containing 90 to 99% by weight of a synthetic ester according to claims 1 to 5 and 1 to 10% by weight of additives.
 7. A hydraulic oil as claimed in claim 6, characterized in that it contains additives selected from the classes of antioxidants, extreme-pressure and anti-wear additives, corrosion inhibitors, demulsifiers and/or defoamers.
 8. A hydraulic oil as claimed in claims 6 and 7, characterized in that it contains an additive which in turn contains alkylamines, tolyltriazole, alkyl phosphate and/or succinic acid semiester.
 9. A hydraulic oil as claimed in claim 8, characterized in that it contains the additive in quantities of 0.5 to 2.0% by weight.
 10. The use of the synthetic esters claimed in claims 1 to 5 as basic fluid in hydraulic oils.
 11. The use of an additive mixture containing alkylamines, tolyltriazole, alkyl phosphate and succinic acid semiester for improving the oxidation stability of the synthetic polyolesters claimed in claim
 1. 